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Hybridization of geothermal and biomass resources for power generation is a potential step in the increased commercial utilization of biomass gasifiers in New Zealand. There is generally an increase in the power generation efficiency by hybridizing two power generation technologies. Hybrid power generation also reduces the capital costs for power plants, due to shared equipment. The Taupo Volcanic Zone is home to a large geothermal reservoir, is proximate to the largest area for commercial forestry in New Zealand, and is therefore a promising location for hybridizing geothermal and biomass resources. The Wairakei Power Plant has been used to generate power for over 50 years, but declining reservoir enthalpy and the preferential use of geothermal steam at other geothermal power plants causes there to be unused power generation capacity.

This study examines the potential for integration of biomass gasification as an additional fuel source in the Wairakei Power Plant so as to increase power generation. Possible approaches to integrating syngas firing into the steam system were investigated and the derived hybrid configurations modelled using the heat and material balance software UniSim to estimate the additional power generation possible. Experimental work was limited to measurements on steam condensate in the existing geothermal systems so as to establish steam purity and required clean-up approaches to utilize steam condensate as a source of boiler feed water. This study addresses some of the practical problems related to silica carryover and plant integration so to allow the utilization of biomass synthesis gas to directly heat geothermal steam.

Four hybrid configurations were investigated in order to generate additional power by integrating a biomass gasifier and syngas fired heating to the Wairakei Geothermal System:
- Superheating of geothermal steam for more efficient power generation
- Syngas fired heating used to boil condensate available on site for additional steam generation
- Boiling separated geothermal water immediately after the first separation stage
- Heating of separated geothermal water available at Wairakei to increase power generation at an existing binary power plant

The performance of the dual fluidized bed gasifier was seen to achieve cold gas efficiencies as high as 84% based on the lower heating value of the landing residue feed and the generated syngas. However, this does not include the thermal input of steam used as the gasification agent, as geothermal steam generated on the Wairakei Geothermal Field was used to satisfy this steam requirement. Flue gas from the biomass gasifier and the combustion of syngas on site was used as the drying agent to dry the wet wood chips prior to these wood chips being introduced to the gasifier.

It was found that the geothermal steam supply to Wairakei greatly impacts the power generation that is possible the hybrid configurations. Three scenarios for the potential steam supply conditions were created in order to represent the changes in the additional hybrid power generation that is expected to occur with changing reservoir enthalpy:
Scenario 1: Steam supply consistent with that for January 2015 – July 2016. Sporadic bypassing of turbines by intermediate pressure steam occurs, all steam turbines at Wairakei are fully loaded.
Scenario 2: Reduced steam supply to Wairakei. No steam bypassing occurs, all steam turbines at Wairakei are fully loaded.
Scenario 3: Further reduced steam supply to Wairakei. No steam bypassing occurs, but not all steam turbines are fully loaded. Additional generated steam may be used in partially loaded steam turbines to increase power generation.

Capital cost estimation and an economic evaluation was performed for the proposed hybrid plants in order to quantify the financial implications of implementing the hybrid configurations for each of the steam flow scenarios investigated.

In order to modify the 30 MWe mixed pressure geothermal steam turbines to utilize superheated steam, it was found that there would be an estimated 2.6 MWe decrease in the power generation of the turbines when fully loaded. However, as the modified turbines will use less steam compared to the unmodified turbines, there is a net increase in power generation possible, due to the power generation that may be performed using the saved geothermal steam. This configuration was seen to be the most efficient in Scenario 3, where an average additional power generation of 11.9 MWe is possible from a 15 t/h input of wet landing residues. This resulted in a fuel to electricity efficiency of 29.7% based on the lower heating value of the landing residues. The project was, however, expected to lose $27 million over a 30 year hybrid plant life, and required an estimated capital investment of $48 million.

Water testing was performed on several sources of water available on the Wairakei Geothermal System, in order to evaluate suitability as boiler feed water. It was found that the most appropriate source of water was condensate from the Poihipi Rd Power Plant, which has an estimated average of 54 t/h of condensate available for use. A water cleaning process was then designed based on the contaminants present in the water, in order to ensure safe and reliable operation of the boiler. The process modelling revealed that this configuration generated electricity most efficiently in the conditions of Scenario 3, with an average of 6.2 MWe additional power being generated from a 15 t/h input of wet landing residues. The resulting fuel-electricity was calculated at 14.8% based on the lower heating value of the forest residues. There was a projected loss of $32 million from the implementation of this project over a 30 year hybrid plant life, requiring a $9.6 million investment.

It was found that boiling separated geothermal water after the first separation stage resulted in a decrease in the metals being discharged into the Waikato River due to an increased proportion of the metals being reinjected into the geothermal reservoir. It was found that power could be generated most efficiently in the conditions of Scenario 3, using the additional steam created from boiling the separated geothermal water. An estimated 6.8 MWe of additional electricity could be generated using an input of 15 t/h of wet landing residues, resulting in a fuel to electricity efficiency of 16.2% based on the lower heating value of the landing residues. The project was expected to lose $27 million for a 30 year plant life, and required a capital investment of $8.3 million.

The Wairakei Binary Plant is designed to generate an average of 15 MWe, however an average generation of approximately 13 MWe has been observed for the plant, this is attributed to the flowrate of separated geothermal water being lower than the plant was designed for. In order to supplement the separated geothermal water flow to the Wairakei Binary Plant, the additional geothermal water was expected to require heating in order to avoid increasing silica scaling in the Binary Plant. The heating of additional separated geothermal water for use in the Wairakei Binary Plant was seen to have the highest efficiency in Scenario 1. An increase of 1.4 MWe was expected using a 3.4 t/h average wet wood input, resulting in a fuel to electricity efficiency of 15.5%, based on the lower heating value of the wet landing residues. This project was expected to lose $17 million over the 30 year life of the hybrid plant, and require a capital investment of $7.8 million.

The poor economics associated with implementing any of the hybrid configurations are attributed both to the design constraints of retrofitting the hybrid configurations, and the relatively high cost of the landing residues. It was initially thought that the close proximity and availability of forestry landing residues would result in viable options for boosting geothermal power generation using syngas fired heating. However, due to the inefficiencies associated with retrofitting the hybrid configurations to an existing geothermal plant, and the relatively low sale price of power; the delivered cost of the forest residues was seen to exceed the value of the additional power generation in most cases. It is therefore recommended that the integration of a gasifier into a new geothermal plant from the design stage, and alternative, cheaper, feedstocks for gasification be investigated. It is also believed that the generation and sale of liquid biofuels using geothermal steam as an input to a gasifier may prove to be profitable.